The present invention relates to a planar waveguide having an integral focusing element and suitable grating to effect multiplexing/demultiplexing of optical signals.
Wave length division multiplexing (WDM) is a technique used to spread information carrying signals over a large number of optical frequency channels. This technique can be used to increase transmission capacity over a given optical length, or to direct different channels to different destinations. The use of WDM systems can also be deployed to increase the transmission capacity of optical lengths operating over large distances. In a typical fiber optic link, in an input fiber carrying wave lengths xcex1, . . . xcexn, such a multiplexed system is demultiplexed with the signals at the various wave lengths routed to different destinations. This is carried out with a demultiplexer. In the opposite case, a number of optical fibers having individual wavelengths xcex1, . . . xcexn, where fiber one carries wavelength xcex1 fiber two carries wavelength xcex2, and so forth. A multiplexing element such as a grating will properly direct the individual wavelengths into one output fiber. The device utilized in effecting the multiplexing can consist of an optical element and a diffraction grating. Diffraction gratings are, in their simplest form, arrays of diffractive elements, either apertures or obstacles which have the effect of producing periodic alterations in the phase, amplitude, or both of an emergent wave. A common grating for effecting specular reflection is a blazed grating. Such a grating is one in which ruling grooves having a controlled shape are used to effect a particular interference pattern. Blazed planar gratings with nearly rectangular grooves are often mounted so that the incident propagation vector is substantially normal to either one of the groove faces. Put another way, the incident propagation vector is parallel to the normal to one of the groove faces and the collimation of incident radiation to achieve normal incidence to the grating is done by a collimating lens. This condition is known as autocollimation, and the angular dispersion of such a grating is inversely proportional to the wavelengths of light. Relatively straight forward analysis of the conditions of autocollimation can yield the chromatic resolving power of such a spectrometer set forth by the Littrow grating described above. In most applications, bulk optics is employed to effect such a grating system to effect multiplexing/demultiplexing of an optical signal.
As stated above, optical fibers are a primary vehicle to effect optical communications. However, integrated optical circuits are particularly convenient and efficient to effect various functions to include switching and, in the context of the present disclosure, multiplexing and demultiplexing. To this end, integrated systems which have optical elements and necessary devices such as gratings can be carried out in planar waveguides. To this end, U.S. Pat. No. 5,412,744 to Dragone and U.S. Pat. No. 5,243,672 also to Dragone, the disclosures of which are specifically incorporated herein by reference, disclose planar waveguides used to effect multiplexing and demultiplexing. As can be appreciated from a review of these patents, the system utilized to effect the wavelength selectivity is relatively complex and therefore difficult to manufacture. In addition, U.S. Pat. No. 5,500,910 to Boudreau, et al. discloses an integrated optic multiplexing/demultiplexing device for use with optical wave guides. While the system to Boudreau, et al. is less complicated than many integrated optic multiplexers and demultiplexers, it is still relatively complex and thereby difficult and expensive to manufacture.
Accordingly, what is needed, is an integrated optic multiplexing scheme which incorporates simplicity and thereby reduced cost of manufacture as well as readily available techniques to effect manufacture.
The present invention relates to a planar waveguide structure having a grating element fabricated thereon as well as a focusing element. In the preferred embodiment of the present invention, a Littrow or other blazed grating is disposed at one end of the planar waveguide and a collimating lens element is disposed in the waveguide to collimate the light so that an autocollimation grating as described above is effected in an integrated form. The lens element of the present disclosure is fabricated by laser ablating through the cladding and core layers so that an air lens is constructed in the planar grating. To this end, the collimator of the planar waveguide is preferably laser micromachined and an air lens is disposed therein in a selected area. Because the index of refraction of air is less than that of the polymer core wave guide, a geometric shape opposite that of what is conventional in optical systems is effected. To this end, a lens takes light from a point source and collimates it. On the other hand the same lens will act as a converging lens for collimated light. In the present example, because the indices of refraction are reversed, a concave lens in air is used. This follows from standard paraxial theory of geometric optics, well known to one of ordinary skill in the art.
In an exemplary embodiment of the present disclosure, an input optical fiber having a multiplexed signal wavelengths xcex1 and xcex2 is coupled to the planar wave guide of the present disclosure. This light impinges thereafter on the air lens and thereby collimated. The collimated light from the input fiber is impingent on the blazed grating disposed on the opposite surface of the planar waveguide, and is spatially separated by wave length according to well-known diffraction theory as discussed above. The light is then reflected from the grating and appropriately focused by the lens element which now acts as a converging element to selectively dispose the spacially separated wave length of light to output fibers at the same end of the wave guide as is the input fiber.
It is an object of the present invention to have an integrated optical multiplexer/demultiplexer in a planar waveguide structure.
It is a feature of the present invention to have a lens element in the waveguide structure, with the lens being a void or other medium having an index of refraction less than that of the core layer of the waveguide in the waveguide layers.
It is a further feature of the present invention to have a diffraction grating for wavelength coupling/separation fabricated on the planar waveguide.
It is an advantage of the present invention to have a readily manufactured, simple structure to effect multiplexing and demultiplexing in an optical waveguide system.